A semiconductor wafer, such as a silicon or a gallium arsenide wafer, generally has a substrate surface on which one or more integrated circuits is formed. The substrate surface is desirably as flat, or planar, as possible before the surface is processed to form the integrated circuits. A variety of semiconductor processes are used to form the integrated circuits on the flat surface, during which the wafer takes on a defined topography. The topography is subsequently planarized, because an irregular surface, or a surface having imperfections, seriously impedes subsequent fabrication processes, such as photolithography. Thus, it is necessary to polish the wafer surface to render it as planar or uniform as possible and to remove surface imperfections.
Chemical-mechanical polishing or planarization (CMP) processes are well-known. See, for example, Chemical Mechanical Polishing in Silicon Processing, Semiconductors and Semimetals, Vol. 62, Edited by Li, S. et al., which is expressly incorporated herein by reference. Also directly incorporated by reference for all purposes are commonly assigned:                U.S. Pat. No. 5,891,205 to Picardi et al., which issued on Apr. 6, 1999, entitled Chemical Mechanical Polishing Composition;        U.S. Pat. No. 5,981,454 to Small, which issued on Nov. 9, 1999, entitled Post Clean Treatment Composition Comprising An Organic Acid And Hydroxylamine;        U.S. Pat. No. 6,117,783 to Small et al., which issued on Sep. 12, 2000, entitled Chemical Mechanical Polishing Composition And Process;        U.S. Pat. No. 6,156,661 to Small, which issued on Dec. 5, 2000, entitled Post Clean Treatment;        U.S. Pat. No. 6,235,693 to Cheng et al., which issued on May 22, 2001, entitled Lactam Compositions For Cleaning Organic And Plasma Etched Residues For Semiconductor Devices;        U.S. Pat. No. 6,248,704 to Small et al., which issued on Jun. 19, 2001, entitled Compositions For Cleaning Organic And Plasma Etched Residues For Semiconductors Devices;        U.S. Pat. No. 6,251,150 to Small et al., which issued on Jun. 26, 2001, entitled Slurry Composition And Method Of Chemical Mechanical Polishing Using Same;        U.S. Pat. No. 6,313,039 to Small et al., which issued on Nov. 6, 2001, entitled Chemical Mechanical Polishing Composition And Process; and        U.S. Pat. No. 6,498,131 to Small et al., which issued on Dec. 24, 2002, entitled Composition For Cleaning Chemical Mechanical Planarization Apparatus.        
CMP processes are commonly used to polish or “planarize” the surfaces of wafers at various stages of fabrication to improve wafer yield, performance and reliability. In CMP, typically the wafer is held in place on a mount using negative pressure, such as vacuum, or hydrostatic or pneumatic pressure. The mount is typically situated over a polishing pad. CMP generally involves applying a polishing composition or slurry to the polishing pad, establishing pressure-contact between the composition- or slurry-coated wafer surface and the polishing pad while providing relative motion, typically rotational or orbital motion, between the wafer surface and the polishing pad.
The polishing composition typically contains an abrasive material, such as silica, ceria, and/or alumina particles, in an acidic, neutral, or basic solution. Merely by way of example, a polishing composition useful in the CMP of tungsten material on a substrate may contain abrasive alumina, also called aluminum oxide, an oxidizing agent such as hydrogen peroxide, and either potassium hydroxide or ammonium hydroxide. A CMP process employing such a polishing composition may provide a predictable rate of polishing, while largely preserving desirable features on the wafer surface.
For such a semiconductor wafer, a typical CMP process involves polishing the metal in a controlled manner to preferentially etch certain conductors, insulators or both over the the oxide beneath the metal, such that the metal is substantially coplanar with the oxide and remains in the grooves or stud vias of the oxide. After CMP, the substantially coplanar surface is ready for further processing. CMP is currently the primary method used to polish or “planarize” wafers in back end of the line (BEOL) processes.
Semiconductor fabrication processes such as photolithography have evolved significantly, such that advanced devices having very fine oxide, metal, and other surface features, with sub-0.25 micron geometries (such as 0.18 micron or less), are now being made. Process tolerances are necessarily tighter for these advanced devices, calling for improvements in CMP technology to obtain desired material removal rates while minimizing wafer defects or damage. A variety of approaches have been taken in an effort to improve CMP processes to improve planarity.
On the other hand, economic forces are requiring the use of faster processing. One approach has involved increasing the downward pressure on the wafer carrier in order to increase material removal rates. This approach is generally disfavored as the requisite downward pressure is considered too high and too likely to cause wafer damage, such as scratching, delamination, or destruction of material layers on the wafer. When the wafer is fragile, as is generally the case with substrates layered with films such as porous films having a low dielectric constant, these damage issues are particularly acute and detrimental in terms of wafer yield and performance. Generally, faster chemical-mechanical polishing results in more defects.
Additional approaches have involved using various protected combinations of oxidizers, chelators, corrosion inhibitors, solvents, and other chemicals in the slurry, various abrasives including for example a zirconium abrasive or mixed abrasives, and/or using point-of-use mixing techniques. These approaches are generally undesirable, as they typically complicate CMP in terms of tooling and process control for example, consume more process time, and/or increase costs.
Another approach has involved increasing the amount of oxidizing agent used in the CMP slurry in an effort to increase chemical removal of targeted material. This approach is largely disfavored as the use of increased amounts of oxidizing agents increase material costs and also detrimentally add to the handling issues and environmental issues associated with many oxidizing agents and also increase costs.
It is generally known that oxidizers admixed in a solution can provide synergistic etching rates. While ferric salts, cerium salts, peroxides, persulfates, or hydroxylamines form the oxidizing capacity of most commercially available CMP slurries, those of ordinary skill in the art have long known that these oxidizers can be admixed with others in this group and also with other oxidizers, and the resulting composition can show synergistic results.
For example, the compositions claimed in U.S. Pat. No. 6,117,783 to Small et al., which claims priority to a provisional application filed Jul. 25, 1996, the contents of which is incorporated herein by reference thereto, claims a CMP slurry having a hydroxylamine compound and hydrogen peroxide, and teaches in the specification that the two have a synergistic effect. U.S. Pat. No. 6,117,783 also claims a CMP slurry having a hydroxylamine compound and ammonium bifluoride. These are mixtures of non-metal-containing oxidizers that provide synergistic results. Similarly, U.S. Pat. No. 5,783,489, the disclosure of which is incorporated herein by reference thereto, discloses an aqueous CMP slurry comprising at least two oxidizing agents, an organic acid and an abrasive having a pH ranging from about 2.0 to about 8.0.
Without being bound to theory, it is believed that certain metal salt oxidizers have a greater oxidizing “probability” than non-metal-containing oxidizers, which may be based at least in part on affinity of the oxidizer to the substrate. Greater affinity enhances the possibility of oxidation but also creates a problem in that the molecule with the greater affinity does not as readily leave the substrate after oxidizing the substrate as other oxidizers. Synergy with metal-containing and non-metal-containing oxidizers may be observed if the other, typically non-metal-containing, oxidizers can oxidize spent oxidizer that is near or on the substrate, such that reaction with the substrate would be fast. Following this line of reasoning, it stands to reason that it is beneficial to have some minimum amount of the metal, to have enough metal-containing oxidizer ions near the surface, but a large excess of the non-metal-containing oxidizer would be beneficial to more quickly re-oxidize the spent metal-containing oxidizer.
Of course, the soluble salt of any metal having multiple oxidation states may be an oxidizer, provided they have the oxidative potential to oxidize the substrate. Metal-containing oxidizers such as permanganate, perchromate, iron salts, aluminum salts, cerium salts, and the like are commonly used in CMP slurries, and synergistic combinations of the metal-containing oxidizers as well as of metal-containing and non-metal-containing oxidizers is also known. CMP of certain metal substrates, for example a copper-containing substrate, will doubtless provide metals, for example cupric and/or cuprous metal ions, in the solution, but these will not oxidize the remaining copper layer. If there are two different metals, however, the oxidized and removed ion of one metal may in turn be an oxidizing metal for another metal, but the amount will be very small.
U.S. Pat. No. 4,959,113, reissued, filed on Jul. 31, 1989, the disclosure of which is incorporated herein by reference thereto, claims synergistic CMP slurries having two or more salts where the cations are selected from ionized elements (i.e., metals) which will not deposit by electroless plating on the metal surface being polished. This patent states “preferred cationic component of the salt is an ionized element from Groups IIA, IIIA, IVA and IVB of the periodic table of elements, as well as zinc, cerium, tin and iron ions . . . (and) an aqueous polishing composition comprising a combination of salts with the water and abrasive agent provides improved polishing of metal surfaces compared to the use of a single salt. Thus, there appears to be a synergistic effect when a combination of two or more salts is used in the polishing composition compared to the use of a single salt.”
One metal-containing oxidizing agent used in CMP is silver nitrate. Silver nitrate and hydrogen peroxide are present in the CMP slurry of U.S. Pat. No. 5,354,490, the contents of which is incorporated herein by reference thereto. Synergy is taught, as the patent stated the silver nitrate converts, at the copper containing metal surface, a solid copper film or a solid copper alloy film into an aqueous phase, while the role of the second oxidizing agent, i.e., hydrogen peroxide, would be to form a copper oxide. The copper oxide would be subsequently removed by the mechanical polishing of the CMP action, such that the addition of the second oxidizing agent can increase the mechanical polishing component of the CMP process.
Another metal-containing oxidizing agent commonly used in CMP is ferric nitrate. U.S. Pat. No. 5,527,423, the contents of which is incorporated herein by reference thereto, teaches a CMP slurry that contains oxidizing components such as mixtures of iron salts and persulfates. Ferric nitrate has been used extensively where tungsten metal or alloys present on the substrate require polishing.
However, ferric nitrate causes metallic contamination of many substrates, including tungsten substrates. Raghunath et al showed in Mechanistic Aspects Of Chemical Mechanical Polishing Of Tungsten Using Ferric Ion Based Alumina Slurries, in the Proceedings of the First International Symposium on Chemical Mechanical Planarization, 1997, that alumina slurries containing ferric salts is conducive to the formation of an insoluble layer of ferrous tungstate on tungsten. The addition of hydrogen peroxide to ferric ion solutions is known. Basak et al., in the same Proceedings of the First International Symposium on Chemical Mechanical Planarization: Proceedings of Chemical Mechanical Planarization in IC Device Manufacturing, 1997, noted that the electrochemical behaviour of tungsten in solutions containing ferric nitrate revealed the presence of ferric ions increases the open circuit potential of W into the regime where oxide films are stable, but anodic currents increased by at least one order of magnitude on addition of hydrogen peroxide.
Some investigators call small quantities of metal-containing oxidizer salts a catalyst as it causes synergistic etching rates when admixed with other oxidizers. See for example U.S. Pat. No. 3,293,093, the disclosure of which is incorporated herein by reference, which teaches a hydrogen peroxide-based etching solution for copper. The patentees noted that many metals, particularly copper ions, “form active metal ions which have been found to have a highly depreciating effect on hydrogen peroxide (so) that it is rapidly exhausted” These investigators wanted to arrest the depreciating effect of metal ions and yet to provide compounds having a catalytic effect on the etch rate of copper. They noted that a small amount of silver ions, and preferably also a small amount of phenacetin, gave enhanced etching and stability. This patent taught a solution having 2-12% hydrogen peroxide and a “catalytic amount” of silver ions, as silver ions are highly effective at improving the etch rate of hydrogen peroxide, and suggests adding silver nitrate salts. A combination of phenacetin and silver ions with acidified hydrogen peroxide exhibits “exceptionally fast etch rates significantly greater than when either additive is used alone.” The patent claims “as little as 10 parts per million” of silver ions is effective, and “about 50-500 parts per million of free silver ion generally represents the preferred amount.” A composition of ammonium persulfate and a mercuric chloride catalyst was also taught in this patent.
Other investigators have also tried to mix oxidizers to achieve synergy. U.S. Pat. No. 5,958,288, the disclosure of which is incorporated herein by reference, suggests limiting the amount of “catalyst” to from about 0.001 to about 2.0 weight percent. This patent describes the catalyst as a compound having multiple oxidation states, and that the catalyst must be able to shuffle electrons efficiently and rapidly between the oxidizer and metal substrate surface. While this broad description of a catalyst encompasses any oxidizer, including any metal salt, the only catalysts described therein are metal salt compounds of Ag, Co, Cr, Cu, Fe, Mo, Mn, Nb, Ni, Os, Pd, Ru, Sn, Ti, and V, most preferably a compound of iron, copper, and/or silver. This patent defines the oxidizing agent to have an electrochemical potential greater than the electrochemical potential necessary to oxidize the catalyst, including but are not limited to periodic acid, periodate salts, perbromic acid, perbromate salts, perchloric acid, perchloric salts, perboric acid, and perborate salts and permanganates, as well as bromates, chlorates, chromates, iodates, iodic acid, and cerium (IV) compounds.
As shown in the above-described art, cerium salts are another metal-containing oxidizer. U.S. Pat. No. 4,769,073, the contents of which is incorporated herein by reference thereto, describes cerium-based polishing compositions for polishing organic glass surfaces which comprise ceric oxide, a cerous salt, and, optionally, pyrosilicates or silica. Similarly, U.S. Pat. No. 5,773,364 filed Oct. 21, 1996, the contents of which is incorporated herein by reference thereto, describes a CMP slurry where oxidizers include ferric nitrate or cerium nitrate, and note the problem that metal ions are created as a result of the oxidizing process. Cerium salts can contaminate an exposed surface of a semiconductor wafer which could affect the reliability and functionality of semiconductor devices on the wafer. In addition, these metallic species will coat/stain the CMP equipment which creates particulate problems and reduces the life cycle of the CMP equipment. This in turn causes increased replacement of polishing equipment and greater cost associated with the manufacturing process.
There is another mechanism for synergy that has not been described in the CMP art, but is known in the unrelated environmental clean-up art. A reaction used in environmental remediation systems is Fenton's reaction, where the relatively benign reactants generate a free radical which can cleave even very resistant organic contaminants.
Fenton's reaction is the interaction of hydrogen peroxide with selected transition metals to produce free radicals. The interaction of copper or a ferrous salt iron and hydrogen peroxide to produce a free radical was first observed by Fenton in 1876. The Fenton reaction is the production of free radicals as a byproduct of the oxidation of ferrous ions by hydrogen peroxide. Other metals are known to have special oxygen transfer properties which improve the utility of hydrogen peroxide.
The optimal pH for Fenton's reaction occurs between pH 3 and pH 6, particularly 4 to 5. The drop in efficiency on the basic side is attributed to the transition of iron from a hydrated ferrous ion to a colloidal ferric species which catalytically decomposes the hydrogen peroxide into oxygen and water, without forming hydroxyl radicals. Fenton's reactions where the iron and the hydrogen peroxide are in solution are characterized by an optimal dose range for iron activator. A minimal threshold concentration of 3-15 mg/L Fe which allows the reaction to proceed within a reasonable period of time for the digestion of organic material in wastewater, and generally a ratio of 1 part Fe per 5-25 parts hydrogen peroxide (wt/wt) is most efficient. For a solution containing organic material to be degraded, to obtain efficient Fenton's reaction kinetics, addition of 5% by weight hydrogen peroxide would also require between about 0.2% to 1% ferrous ions in the solution.
It is also known that UV light can enhance the efficiency of Fenton's reaction, and that some activators need actinic radiation to be operative. For example, U.S. Pat. Nos. 6,117,026 and 6,435,947, the disclosure of which is incorporated herein by reference, describe a heterogeneous solid metal oxide catalyst that can be a homogeneous composition of the active catalyst, or the active heterogeneous solid catalyst can be chemically or physically associated with the surface of the preferred abrasive as molecular species, as a small particle or as a monolayer. The solid catalysts are preferably small particles with high surface areas. The solid catalysts should have a mean particle diameter less than about 1 micron and a surface area greater that about 10 m.sup.2/g and less than about 250 m.sup.2/g. It is more preferred that the solid catalysts have a mean particle diameter that is less than about 0.5 microns and most preferably less than about 0.25 microns.
As mentioned in U.S. Pat. Nos. 5,773,364, 4,959,113, and others, there are problems with the metal-containing oxidizers. When a metal-containing oxidizer is admixed with another metal-containing oxidizer, there is a possibility of plating of one of the metals due to the differences in electrochemical potential of the various metals at the various oxidation states, particularly as the solution is consumed during polishing of a substrate. While plating was recognized as problematic in the U.S. Pat. No. 4,959,113, there is a further possibility that as the metal-containing oxidizers change oxidation states, even some “non-plating” combinations may become plating.
Another problem with many metal compounds is that they react with and cause degradation of other oxidizers. When a metal-containing oxidizer is admixed with a non-metal-containing oxidizer, for example hydrogen peroxide in a solution, the two often react in an undesirable fashion, and the oxidizing capacity of the mixture declines rapidly with time. The nature of the reaction can take many forms. For example, ferric nitrate reacts with hydrogen peroxide in CMP formulations at essentially all usable pHs, making the formulation oxidizing capacity fall with time, which complicates polishing since there is a non-uniformity problem, and also causing formation of undesired products. It is known that if the pH is above about 5, iron precipitates as Fe(OH)3 which catalytically decomposes the hydrogen peroxide to oxygen. The mechanism for decomposition at pH below 5 is not known.
Another problem with metal-containing oxidizer salts is that they leave metal contamination on the substrate. This metallic contamination can result in shorts and unwanted conductive properties, along with other problems. Metal contamination was recognized in U.S. Pat. No. 5,445,996, filed May 25, 1993, the contents of which is incorporated herein by reference thereto, describes use of a polishing slurry for polishing and planarizing the semiconductor device that contains less than 100 ppm impurities such as sodium, potassium, and other alkali metals.
Certain metals, such as those with a tendency to plate on or be absorbed on to at least one part of the substrate, are more damaging than other metals. The industry has developed methods to remove a portion of the metallic contamination, for example by: physical desorption by solvents; changing the surface charge with either acids or bases so that Si—OH or M—OH group can be protonated (made positive) in acid or made negative with bases by removing the proton; ion competition, for example removing adsorbed metal ions by adding acid (i.e. ion exchange); subsequent oxidation of metals to change the chemical bonds between the impurities and substrate surface; and subsequent etching the surface, wherein the impurity and a certain thickness of the substrate surface is removed, as described in U.S. Pat. No. 6,313,039, the contents of which has been incorporated herein by reference. There have been various “post-polishing cleaners” developed to remove metallic contamination, but removal of all undesired metal ions is substantially beyond the range of cleaners, and as the size of the structures continues to decrease, even a very small number of metallic atoms deposited on a surface will result in undesired shorts or current leakage.
Additionally, metal ion-containing fluids are often environmentally undesirable and expensive treatment may be needed prior to waste disposal of used product.
Therefore, despite the known (and heretofore unknown) advantages of having multiple oxidizers, for example a metal-containing oxidizer admixed with either another metal-containing oxidizer or with a non-metal-containing oxidizer, there has been a tendency in the industry to reduce the amount of metal ions in CMP slurries. For example, Rodel, a large commercial manufacturer of CMP slurries that at point of use are designed to be used with non-metal-containing oxidizers such as peroxides and persulfates, had about 30 ppm of metals, primarily iron, in the liquid portion of an MSW1000™ slurry produced in 1995. While this iron would have functioned as a promoter, it is likely the iron was in the solution as a result of impurities. Later generations of Rodel slurries, for example the Rodel MSW1500™ slurry that was sold in 2002, has only 4.2 ppm iron.
Another method of reducing metallic contamination is to use sequential CMP polishing steps using sequential formulations that have decreasing amounts of metal, so that metal deposited from earlier formulations in a CMP process are removed by CMP with subsequent formulations that are metal-free. For example, the newest generation of Rodel CMP slurries, the MSW2000™, has a first formulation (A) having 12 ppm Fe, and a second formulation (B) that has less than 0.3 ppm Fe. However, use of sequential formulations adds additional costs to processing, as well as adding complexity to the required equipment. Cabot Corporation, another large commercial manufacturer of CMP slurries, now sells several high-purity, nonmetal-based CMP slurries for tungsten, such as the Semi-Sperse® W2000 and the Semi-Sperse® W2585 slurries, claiming that the slurries eliminate the secondary-polishing steps associated with existing tungsten slurries.
EKC Technology/Dupont Electronic Technologies, another large commercial manufacturer of CMP slurries, sells several high-purity, non-metal-based CMP slurries for tungsten, for example the MicroPlanar® CMP3550™/MicroPlanar® CMP3510™ slurry, as well as the traditional but effective ferric nitrate as the oxidizer with a post-CMP cleaner to remove metal contaminants.
It is clear that the industry is moving away from metals, for example iron, in the fluids. Also, when iron or other metal-containing formulation is admixed with non-metal-containing oxidizers, the “pot-life” of the formulation is very short, so mixing is generally point-of-use mixing, which complicates CMP processes and equipment and can create start-up problems even after a temporary interruption on the processing.
Further developments in the field of CMP technology are desired.